The invention concerns a magnetically driven pump comprising:
a pump element fitted with a first driven rotor in the form of a wheel mounted to rotate in a body connected to the suction and discharge piping,
a first series of magnets integral with the first rotor,
a driving motor fitted with a drive shaft on which is mounted a second driving rotor carrying a second series of magnets., both series of magnets being laid out concentrically to provide rotation magnetic coupling,
and a sealing device having a fixed partition extending in the gap between both series of magnets while providing tight separation between the pump element and the motor.
The sealing partition is particularly important when the pumped liquid exhibits a corrosive nature, which is frequently the case in chemistry or electroplating.
For these applications, that do not lend themselves to stoppages readily, it should be noted that maintenance operations must be as short as possible, if not suppressed altogether.
The pumps used currently can be classified in two categories:
pumps fitted with a sealing gasket (FIG. 1), comprising friction pieces mounted fixed on the one hand, and rotating on the other hand, whereas the nature of the materials considered and the quality of their surface condition enable to obtain satisfactory sealing properties;
magnetically driven pumps (FIG. 2), which have been designed to remedy the shortcomings mentioned above, whereas sealing is provided not by friction pieces any longer, but by a continuous partition. On either side of that partition, there are a driving rotor linked to the motor, and a driven rotor linked to the wheel of the pump. Both rotors concern magnets laid out so that they provide a magnetic coupling between both rotors.
On FIG. 1, the motor 1 is linked to the centrifugal wheel 2 by an axis 3 and a rigid coupling device 4. The wheel 2 rotates in the pump body 5 that communicates with the suction 6 and discharge 7 piping. The pump body is sealed to the passageway of the spindle 3 thanks to the friction joint 8.
The first fault that can be ascribed to that type of pump is that the friction pieces forming that joint are exposed to wear and that they must therefore be replaced periodically, which involves downtimes for maintenance purposes. This replacement operation is even trickier since the joint 8 is situated in a hardly accessible zone.
A second potential fault is the sealing effect properly speaking that cannot be guaranteed perfectly, because of the small surface defects that may be encountered on friction bearing faces, and of the inevitable formation of a liquid film between these surfaces.
FIG. 2 shows the motor 11, the wheel 12, the pump body 15, the suction 16 and discharge 17 piping. Sealing is here provided by the continuous partition 18 assembled rigidly and hermetically between the pump body 15 and the spacer 19 providing the necessary link with the flange of the motor 11. On the spindle of the motor 11 is mounted rigidly the driving rotor 20 in which is inserted, for example by duplicate molding, a series of magnets 21.
The driven rotor 22 integral with the wheel 12 is equipped with a series of magnets 23. The magnets 21, 23 are organized so that a driving side north pole faces a driven side south pole, and conversely. There is thus provided a magnetic coupling without any mechanical contacts, a coupling that must therefore be sufficient to sustain the maximum torque absorbed by the wheel without stalling.
Good efficiency of the coupling requires that the gap between both families of magnets should be as little as possible. This gap being formed by the thickness of the partition 18 and by the plays either side of the said partition, it appears that the following targets should be reached:
to minimize the thickness of the partition, which implies that it should not be stressed excessively from a mechanical viewpoint and/or that it is made of a material with good stiffness;
to reduce the plays, which implies good dimension stability of the parts affected, as well as their positioning.
About the first item, there may be a contradiction between mechanical stability of the partition and its chemical compatibility with the pumped liquid, with which it is in contact by its internal face.
A solution widely used consists in realizing that partition by juxtaposing two materials:
externally, an a magnetic metal portion providing precision and stiffness.
internally, a portion made of chemically compatible synthetic material.
Such arrangement solves the problem rather well, but it exhibits two significant shortcomings:
an increase in thickness, and hence in the gap.
the presence of Foucault currents in the metal partition, these currents being induced by the rotation of the flux of the magnets. These Foucault currents form a heating source that may become prohibitive, notably for large plants.
As regards the second item, i.e. the plays, the positioning and rotation guiding device of the wheel 12 according to FIG. 2, is formed:
of a fixed spindle 24, mounted with stiffness and precision on the partition 18,
of a fixed ring 25 integral with the spindle 24,
and of a rotating ring 26 integral with the wheel 12.
The quality and the arrangement of the rings 24, 25 are obviously essential to the stability of the pump, with notably:
as generous sizing as possible of the surfaces in contact.
judicious selection of the materials (ceramic, silicon carbide, graphite . . . ) and of their surface condition.
judicious use of the pumped liquid to ensure lubrication
as good an evacuation as possible of the calories generated by friction.
Inspection of FIG. 2 shows clearly the shortcomings inherent to the assembly of the spindle 24 as regards precision, hence control of the plays. Its positioning with respect to the axis of the motor (with which it must be aligned theoretically) is provided indeed by two parts whereof the precision and stiffness may be problematic: the spacer 19 and especially the partition 18. It has been observed that said partition must be thin enough to go through the gap and not generate too many Foucault currents.
It will be therefore quite difficult to embed the spindle 24 correctly. It has been suggested to improve the mechanical stability while providing an additional bearing at the other end of the wheel, but that solution increases the complexity notably without solving the problem adequately.
Finally, as regards the evacuation of the calories absorbed by the spindle 24, it should be noted that it must be performed through the partition 18 that does not lend itself very well to that process, still because of its excessive thinness.
The document FR-A-2311201 describes a magnetically driven pump, wherein the turbine is equipped with a magnetic core and is driven by the magnetic crown through a tight partition. The rotating turbine is supported by a fixed shaft, which is guided by a pair of bearings on the magnetic crown in connection with the motor shaft. The presence of the bearings on top of the output bearing of the motor shaft causes the assembly to overhang significantly and to be embedded even more. The axial space requirements of the pump are important, and the positioning of the shaft of the turbine does not enable to obtain perfect alignment.
The object of this patent is to suggest a solution enabling to remedy the above shortcomings, i.e. ensure on the one hand perfect centering of the rotational axis of the wheel of the pump, while relieving the sealing partition from that function, and on the other hand while seeking efficient evacuation of the calories toward a cooling element.
The pump according to the invention is characterized in that:
the first driven rotor revolves on a cylindrical shoulder whereof the positioning and support are provided by an axial connection piece extending in the continuation of the shaft of the motor,
a female cylindrical bearing serves as a concentric recess for the connection piece in order to provide mechanical support and accurate centering of the partition and of the first driven rotor.
The spindle of the motor is continued advantageously by a sufficient length to enable said spindle to engage inside the driven rotor. It results that the spindle of the motor encompasses the spindle of the wheel which, from fixed becomes rotating. The purpose is obviously not to obtain that rotation, but to provide the first driven rotor with a stiff support that is aligned perfectly with the motor spindle.
According to a preferred embodiment, the first driven rotor comprises a second ring that rotates on a first ring integral with the fixed partition. The bearing integral with the partition comprises at least one self-lubricating ring forming a thermal bridge for the evacuation of the calories generated by the rotation of the first driven rotor towards the heater formed by the motor shaft.
As the sealing partition should remain undisturbed, its shape must be made slightly more complex so that it may run around the extended connection piece, which belongs to the zone outside the pumping circuit, whereas the spindle 24 according to FIG. 2 of the previous art was part of the internal zone.
On top of the cylindrical portion in the gap, the partition should therefore exhibit a second cylindrical portion engaging on the end of the motor spindle, with interposition of a friction bushing, made for example of self-lubricating material.
As that spindle now ensures the positioning of the driven rotor with the requested precision and stiffness, that function need not be fulfilled by the sealing partition, which can be consequently be made lighter. In the easiest embodiment, that partition can be made of a single part, in a material chemically compatible with the pumped liquid.
It should be noted however that the part must remain capable to sustain the pressure of the liquid present around the driven rotor, a pressure that is significant since it can be close to the discharge pressure of the pump. In case when that pressure is high and when there is no material chemically compatible exhibiting sufficient mechanical stability, one can be led to consider a composite partition comprising a mechanical resistant external envelope and a chemically compatible internal envelope.
One is not however exposed to the same constraints as with conventional pumps corresponding to FIG. 2. Indeed, stability to internal pressure is much easier to ensure than stiffness and precision.
The external envelope can then be much thinner, which enables to contemplate its realization:
either in a magnetic metal, as in the conventional solution, but by selecting very small thickness, which brings the losses caused by Foucault currents to its acceptable value;
or in synthetic material (loaded polyamide or polycarbonate for example), which calls for moderate increase in the gap, but suppresses Foucault currents totally.
As regards the evacuation of the calories generated by the rotation, the arrangement described above exhibits an obvious advantage, inasmuch as it produces a thermal bridge with large cross-section and little thickness between the bearing of the driven rotor and the shaft of the motor. Obviously, this advantage is mitigated by the fact that the calories produced by the rotation of the additional connection piece of the shaft of the motor in its own bearing should be evacuated, but this position lies outside the reach of the pumped liquid, which enables to use conventional mechanical components, whereof the output is excellent.
According to another characteristic of the invention, the sealing partition is formed for better chemical compatibility, whereas precision and mechanical stability are ensured by an additional part matching partially the shape of the partition, and realized in a material with good mechanical stability. The additional part can be realized in metal alloy, notably stainless steel, and comprises a ferrule inserted in the gap provided between both series of magnets. The thickness of the ferrule is smaller than that of the envelope of the partition.